This invention is generally directed to a threaded male fastener member, or other threaded member, having a novel point which facilitates engagement of the fastener member in a threaded bore of a correspondingly threaded female member and prevents the member from cross-threading upon engagement, the method of assembly thereof, and the method of forming the threaded member. Either the member or the female member may be driven, the other member being stationary during the assembly.
During automated or manual assembly of an externally threaded fastener member with an internally threaded member, such as a nut, a tapped bore in a workpiece, or an internally threaded rivet staked to a workpiece, cross-threading can occur at assembly. Cross-threading is an unacceptable condition.
To obtain proper alignment and engagement, the external thread on the fastener member engages with the internal thread in the female member and the external thread then follows the helix path of the internal thread. The mating of the helix angle of the internal and external threads provides an inclined plane which converts input torque into clamp force in the joint.
Cross-threading occurs when a threaded member is engaged with a female member and the threads are not properly aligned when the threads on the threaded member and in the female member first engage. If the external threads on the threaded member and the internal threads in the female member are not properly aligned at the start of the threading operation, the external thread tends to cross over the crest of the internal thread which can produce deformation of the threads and binding or seizing up of the components before the desired depth of engaging is obtained. Unless the threaded member or the female member are manually realigned at this point, the external thread will wedge against the internal thread thus preventing assembly and/or damaging the threads. Cross-threading is a common occurrence in prior art fastener members, and extremely troublesome in automated assembly operations.
In an automated assembly situation, the occurrence of cross-threading can severely affect productivity. In some cases, assembly workers will have to manually engage the fastener member and the female member for a couple of threads, then apply the automated equipment to these pre-aligned fastener members.
Prior art fastener members which have attempted to prevent cross-threading within a tapped bore are provided with various designs. These prior art designs generally involve an asymmetrical point or entry end on the fastener member, with only a portion thereof on the asymmetrical end point or entry end. Examples of prior art fastener members include a "P" Point, a MAT Point, a CTP Point which is disclosed in U.S. Pat. Nos. 4,981,406 and 5,419,667, an ACT Point which is disclosed in U.S. Pat. Nos. 4,789,288 and 4,915,560, and a TRU-START which is disclosed in U.S. Pat. No. 5,064,327.
These prior art fastener members have point lengths, which is defined as the distance from the first full thread to the end of the fastener member, which are substantially longer than a standard machine screw point length. Approximate point lengths for these prior art fastener members are as follows:
MATT thread=10 pitch PA1 GMP-Point=7.2 pitch PA1 TRU-START, short dog point=5.6 pitch PA1 Machine screw=2 pitch
Many of the current anti-cross-threading points rely on the point length to provide alignment of the fastener member with respect to the internally threaded hole. Many of the existing machine screw applications cannot tolerate additional point length due to interference in blind tapped holes or other mating components because most applications are replacing standard machine screws. This is particularly important in small business equipment applications, such as, hard drives, cell phones, etc. Also, the additional point length adds additional weight versus a standard machine screw which can be significant in automotive applications.
Applicant's co-pending application, Ser. No. 08/916,164, filed on Aug. 21, 1997 and entitled "Fastener With Anti-Cross-Threading Point and Method of Assembly" greatly reduces the possibility of cross-threading and no-start problems. Applicant has found, however, that in some applications, such as blind holes, the point length of the fastener member disclosed in Ser. No. 08/916,164 is too long making it unsuitable for use in this applications. This present invention overcomes this problem. In addition, in the applications which involve the use of very thin threaded holes, such as in "Tinnerman" type push-on clips, the fastener member disclosed Ser. No. 08/916,164, which uses a long, bulbous point to align itself, is not suitable for use in these applications.
It is desirable to obtain proper alignment, eliminate cross-threading and to provide a fastener member which can be used in thin threaded holes. The fastener member of the present invention provides such a member. The present fastener member minimizes the problems found in the prior art by several novel improvements to prior art fastener members. The structural features of the fastener member of the present invention and manner whereby these features function to attain the improved performance discussed above will become apparent from the discussion as follows, especially with respect to the drawings.